net: mdio: of: fix potential NULL pointer derefernce

of_find_mii_timestamper() returns NULL if no timestamper is found.
Therefore, guard the unregister_mii_timestamper() calls.

Fixes: 1dca22b1 ("net: mdio: of: Register discovered MII time stampers.")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/of/of_mdio.c

@@ -81,12 +81,14 @@ static int of_mdiobus_register_phy(struct mii_bus *mdio,
 	else
 		phy = get_phy_device(mdio, addr, is_c45);
 	if (IS_ERR(phy)) {
+		if (mii_ts)
 			unregister_mii_timestamper(mii_ts);
 		return PTR_ERR(phy);
 	}
 
 	rc = of_irq_get(child, 0);
 	if (rc == -EPROBE_DEFER) {
+		if (mii_ts)
 			unregister_mii_timestamper(mii_ts);
 		phy_device_free(phy);
 		return rc;
@@ -116,6 +118,7 @@ static int of_mdiobus_register_phy(struct mii_bus *mdio,
 	 * register it */
 	rc = phy_device_register(phy);
 	if (rc) {
+		if (mii_ts)
 			unregister_mii_timestamper(mii_ts);
 		phy_device_free(phy);
 		of_node_put(child);